Inside Carbon Nanotubes: SALVE TEM reveals Graphene Ribbons in the Subnanometer Range

January 01, 2021 - Graphene nanoribbons (GNRs) are extremely interesting for nanoscale electronics and devices. However, the growth of high-quality GNRs with well-defined widths is still remaining a challenge. Furthermore, the analysis of the electronic properties of GNRs has remained challenging. Scientists from the Sun Yat-sen University in China, the Universities of Vienna, Budapest and Ulm in Europe and the National Institute of Advanced Industrial Science and Technology in Japan could now synthesize well-defined sub-nanometer GNRs inside carbon nanotubes (CNTs) and measure their electronic properties.

Thanks to the unique electronic properties of graphene [1], it is possible to create a band gap by reducing the width of graphene bands into the sub-nanometer range. The size of the band gap increases with the decreasing width of the GNR [2]. Such structures have interesting applications for future electronic devices [3]. At present, most of the common stripes are of the armchair type. The edge of the ribbons consists of coaxial carbon pairs aligned parallel to the direction of the ribbon axis.

There are many possibilities to further change the properties of the GNR, [4] z. B. by changing their edges. In general, the carbons at the edge of the ribbons are saturated with hydrogen. But by selecting special molecules, by bandgap technology [5], or by constructing band heterojunctions [6] new unusual properties can be created. Recently, GNRs also became relevant for applications in photocatalytic hydrogen generation [7]. Hybrid structures of encapsulated bands and a semiconducting SWCNT with a different band gap can also be very interesting.

Filling and carrying out subsequent chemical reactions in single-walled carbon nanotubes (SWCNTs) is a promising technique for producing nanoscale materials [8]. The European-Asian team of scientists has now shown that this method can be used to produce GNRs with a precisely defined width in the sub-nanometer range. The width of the GNRs is depending on the diameter of the SWCNTs.

The growth of GNRs in SWCNTs was shown by first filling the SWCNTs with flat precursor molecules such as corones [9, 10] or other PAHs [11] and then transforming them at elevated temperature. Although these first results were very promising because of the overlapping electronic transitions of the carbon nanotubes themselves. In order to avoid this overlap, the otherwise flawless tubes had to be functionalized [12].

Aberration Corrected High Resolution Transmission Electron Microscopy (AC-HRTEM)

The aberration-corrected high-resolution electron microscope (AC-HRTEM) enables these interesting subnanometer-sized two-dimensional objects to be examined more closely. One can depict the CNT and also objects inside with atomic resolution. In addition, using electron diffraction, one can determine other properties such as stacking and tube chirality. The electron beam can also be used to transform the band structure over time, which makes further interesting investigations possible.

“The AC-HRTEM can also differentiate between flat and tubular structures within the CNT,” says Ute Kaiser, who heads low voltage transmission electron microscopy at Ulm University. "But how can one determine the exact atomic structure and especially the electronic properties of the GNR?"

The new study now focused on enabling a simple and scalable approach for the controlled growth of various well-defined graphene nanoribbons in the sub-nanometer range, including the quantification of their electronic and structural parameters. The time-dependent AC-HRTEM data were used to show that the observed species are flat objects that twist under the influence of the electron beam and cannot be identified as inner nanotubes. Raman scattering combined with first principle calculations and resonance excitation analysis was then used to identify the structure of GNR.

Examples for the TEM images are depicted in Figure 1 and for the Raman images in Box 1-3. Significant intensity difference can be observed between the edge of 7AGNR (Figure 1a) and the edge of the inner SWNT in DWNT (Figure 1e). Whereas for the nanoribbons@SWCNT the intensity profile for the inner objects is flat (Figure 1f, top), it exhibits strong maximum at the edge for the inner objects in SWCNT@SWCNT (Figure 1f, center and bottom).

Next, the team explored the possibilities to determine the electronic properties of the GNR within CNT. The standard feature for the observation of encapsulated ribbons in TEM is an alternating pattern of narrow and wide signals from the material inside [11, 13]. It comes from twisting parts of the ligaments induced by electron beams. Figure 2 shows a collection of results from FeCp₂-filled and subsequently transformed SWCNT species as recorded by the SALVE instrument with a Cs/Cc aberration corrector. It demonstrates the twisting of the graphene bands by the electron beam (panels a - e) and provides images of the bands in atomic resolution with corresponding simulations (panels f - j). The AC-HRTEM images (fields f - j) provide clear evidence of the atomic structure of the ribbons, but also show that the nanoribbons are very unstable under the electron beam and defects along the length of the structure are quickly created by the beam. Therefore, these AC-HRTEM images are not representative for the quality of the growing bands and only serve as evidence that the structures are flat bands. The instability of the TEM image with the irradiation time is shown in more detail in Figure 3.

The authors of the study showed the synthesis of graphene nanoribbons with widths of 0.61 and 0.74 nm and excitonic gaps of 1.83 and 2.18 eV by high-temperature vacuum annealing of ferrocene molecules in single-walled carbon nanotubes. They obtained information the information on the precisely defined electronic structure of the growing ribbons from their detailed, wavelength-dependent Raman spectroscopic experiments. Raman scattering, especially the combination with the evaluation of resonance Raman excitation profiles, is an excellent tool for studying the electronic structure of the objects inside the tubes. This is due to the double selectivity of the resonance Raman scattering process. The Raman spectrum is selective with respect to the geometric structure of the GNR, but also selective with respect to its electronic structure. The narrow width of the Raman lines indicates well-defined geometrical structures, and sharp excitation profiles indicate a uniform electronic configuration of the GNRs. Thus, the Raman method enables the electronic structures of the bands to be determined.

The new study may be providing a solution to the long standing problem that bands with a larger or smaller width than those specified here can be grown in targeted SWCNTs and their structural and electronic properties can be studied with AC-HRTEM and Raman scattering thus open up a new field of subnanometer graphene nanoribbon research.

Highlighted Topics

Raman Scattering

Previously, some of the authors reported the observation of a set of Raman lines after thermal conversion of FeCp₂ filled SWCNT [14, 15] but the origin of these lines remained unclear as they did not fit to proper model calculations and the SLAVE³ TEM was in the final phase of the project [16, 17], so not available at that time. Here the authors now identified the origin of the Raman lines which turned out to be very sensitive to transformation temperature. Figure 4a–d depicts this behavior in a plot where Raman intensities are characterized by a color code as a function of Raman frequency and transformation temperature. Raman spectra were normalized to the 2D band of the encapsulating tubes, which is least influenced by the changes induced by the GNR encapsulation. Figure 4a and b depicts the responses measured for 568 nm excitation while Figure 4c and d presents those for 633 nm excitation. At low temperature, only the RBMs (around 200 cm⁻¹) and the G-line of the SWCNTs (around 1600 cm⁻¹) are observed. When increasing the transformation temperature beyond 500°C, depending on the excitation laser used two groups of Raman lines can be observed. They exhibit a maximum Raman intensity between 600 and 700°C, indicated by the horizontal white arrows in the figure. As shown further on, these two groups of lines correspond to the in-tube synthesis of AGNRs with different widths.

The figures in Figure 4 demonstrate very narrow line widths of the order of 10 cm^-1 for the response of the nanoribbons. This linewidth is equivalent to the Raman response of high-quality ribbons grown on Au substrates and subsequently transferred to semiconducting substrates [18]. Such narrow line widths (in combination with a clear resonance profile) can only be observed for nanoribbons with a well-defined width along their length, as width variations would lead to inhomogeneous broadening of the lines or even disappearance of the RBLM mode.

HRTEM and reciprocal space images of graphene and mos2, experiment and simulation are in good agreement.

Figure 4. Raman scattering for temperature dependent transformation. a-d, Color-code maps for Raman line intensities observed after transformation of FeCp₂@SWCNTs as a function of Raman frequency and transformation temperature. The latter was increased in 50°C steps. Vertical white arrows highlight the positions of the main new Raman lines. Horizontal white arrows are located at temperatures of maximum Raman response. Raman spectra in a, b and in c, d were excited with a yellow laser (568 nm) and red laser (633 nm), respectively. e-h, Raman spectra for FeCp₂@SWCNT tuned by transformation temperature to optimized response for the two groups of lines. e, Spectra as observed for yellow laser excitation at 568 nm in the high frequency region, from bottom to top: tubes filled with FeCp₂ (black) and subsequent transformation at 600°C (red), AGNR contribution by subtracting the Raman signals from the nanotubes (blue, dotted), and calculation (green) for 7-AGNR. The Raman line marked by a down-arrow was also subtracted since it originates form 6-AGNR as discussed below. f, Raman signals from low frequency region, from bottom to top: filled tubes, transformed at 600°C, and calculated for 7-AGNR. The arrow marks the response from DWCNTs obtained during the transformation process. g,h Similar spectra as in e,f but for transformation temperatures of 700°C, recorded with 633 nm laser, and as calculated for 6-AGNR.

Raman Excitation Profiles

Raman Analysis

For the analysis of AGNR@SWCNT the RBLMs are particularly important since they exhibit a strong and characteristic response in a frequency region which is free from other Raman lines. In the case of the 7-AGNR@SWCNT the response is unusual since it exhibits several components. Figure 6a shows a zoomed-in Raman map of region R2. Several peaks can be observed in this region, which become also evident when plotting the Raman spectra for two distinct laser excitations (Figure 6b). The main peak (highest intensity) appears at a vibrational frequency of 414 cm⁻¹ for 569.5 nm excitation with an exceptionally small line width of only 7 cm⁻¹. In addition, a shoulder can be observed around 400 cm⁻¹, which is considerably broader (13 cm⁻¹) and exhibits dispersion, i.e. line position shifts with changing excitation energy. Such behavior is well known for the d-line in CNTs but also for conjugated polymers like poly-acetylene [19]. It is an indication for defective structures where vibrational frequencies and electronic transitions exhibit some correlation which eventually leads to photo-selective resonance scattering. In our case such structures may be represented by interfaces between extended or short ribbons of different structure and different topology. In such cases new electronic state inside the gap can be created [6]. Also, modified edges resulting in sample inhomogeneity can lead to slightly modified RBLM-modes and thus result in photo-selective resonance scattering. In addition to the shoulder a very weak Raman signal is observed at 375 cm⁻¹ for 506.1 nm excitation and at shorter wavelength of 460 nm a Raman line at 417 cm⁻¹. At even shorter wavelengths of 405 nm excitation a Raman line is observed at 533 cm⁻¹.

Figure 6. Raman scattering for the RBLM of 7-AGNR. a, Blown up Raman map for the frequency region between 350 and 440 cm⁻¹; b, Radial breathing like mode for 7-AGNR as excited with laser lines at 2,45 (blue) and 2,18 eV (red). Full drawn lines are fits from the experimental analysis. c, Resonance profile for the main peak of the RBLM at 414 cm⁻¹ (dots, with errorbars) together with the calculated values (red line); d, RBLMs frequencies versus ribbon width w. Blue stars are the experimental results from this work. Black squares are calculated values. The dashed line corresponds to Vanderscuren et al. [20] who found a relation of the form ω_RBLM=a/√(ω-b) with empirical parameters a and b for the dependence of the RBLM frequency on w, the ribbon width without hydrogen. The dashed line in the figure was evaluated for a and b equal to 527.4 and 210.2, respectively. Red dots are for the additional Raman lines at 533 and 375 cm⁻¹ if they are assigned to 5-AGNR and 8-AGNR, respectively. The value for n = 9 is from a reference [21].

Resource: Kuzmany, H., Shi, L., Martinati, M., Cambré, S., Wenseleers, W., Kürti, J., Koltai, J., Kukucska, G., Cao, K., Kaiser, U., Saito, T. & Pichler, T. Well-defined sub-nanometer graphene ribbons synthesized inside carbon nanotubes. Carbon, 171, 221-229, 10.1016/j.carbon.2020.08.065, see also the supporting information.

References

Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., Grigorieva, I. V., & Firsov, A. A. (2004). Electric field effect in atomically thin carbon films. Science, 306(5696), 666-669.
Son, Y. W., Cohen, M. L., & Louie, S. G. (2006). Energy gaps in graphene nanoribbons. Physical Review Letters, 97(21), 216803.
Bennett, P. B., Pedramrazi, Z., Madani, A., Chen, Y. C., de Oteyza, D. G., Chen, C., de Oteyza, D. G., Chen, C., Fischer, F. R., Crommie, M. F., & Bokor, J. (2013). Bottom-up graphene nanoribbon field-effect transistors. Applied Physics Letters, 103(25), 253114.
Gröning, O., Wang, S., Yao, X., Pignedoli, C. A., Barin, G. B., Daniels, C., Cupo, A., Meunier, V., Feng, X., Narita, A., Müllen, K., Ruffieux, P., & Fasel, R. (2018). Engineering of robust topological quantum phases in graphene nanoribbons. Nature, 560(7717), 209-213.
Verzhbitskiy, I. A., Corato, M. D., Ruini, A., Molinari, E., Narita, A., Hu, Y., Schwab, M. G., Bruna, M., Yoon, D., Milana, S., Feng, X., Müllen, K., Ferrari, A. C., Casiraghi, C., & Prezzi, D. (2016). Raman fingerprints of atomically precise graphene nanoribbons. Nano Letters, 16(6), 3442-3447.
Cao, T., Zhao, F., & Louie, S. G. (2017). Topological phases in graphene nanoribbons: junction states, spin centers, and quantum spin chains. Physical Review Letters, 119(7), 076401.
Akilimali, R., Selopal, G. S., Benetti, D., Mohammadnezhad, M., Zhao, H., Wang, Z. M., Stransfield, B., & Rosei, F. (2020). Graphene nanoribbon-TiO2-quantum dots hybrid photoanode to boost the performance of photoelectrochemical for hydrogen generation. Catalysis Today, 340, 161-169.
Khlobystov, A. N. (2011). Carbon nanotubes: from nano test tube to nano-reactor. ACS Nano, 5(12), 9306-9312.
Chernov, A. I., Fedotov, P. V., Talyzin, A. V., Suarez Lopez, I., Anoshkin, I. V., Nasibulin, A. G., Kauppinen, E. I., & Obraztsova, E. D. (2013). Optical properties of graphene nanoribbons encapsulated in single-walled carbon nanotubes. ACS Nano, 7(7), 6346-6353.
Anoshkin, I. V., Talyzin, A. V., Nasibulin, A. G., Krasheninnikov, A. V., Jiang, H., Nieminen, R. M., & Kauppinen, E. I. (2014). Coronene encapsulation in single‐walled carbon nanotubes: stacked columns, peapods, and nanoribbons. ChemPhysChem, 15(8), 1660-1665.
Chamberlain, T. W., Biskupek, J., Rance, G. A., Chuvilin, A., Alexander, T. J., Bichoutskaia, E., Kaiser, U., & Khlobystov, A. N. (2012). Size, structure, and helical twist of graphene nanoribbons controlled by confinement in carbon nanotubes. ACS Nano, 6(5), 3943-3953.
Lim, H. E., Miyata, Y., Fujihara, M., Okada, S., Liu, Z., Sato, K., Omachi, H., Kitaura, R., Irle, S., Suenaga, K., & Shinohara, H. (2015). Fabrication and optical probing of highly extended, ultrathin graphene nanoribbons in carbon nanotubes. ACS nano, 9(5), 5034-5040.
Chuvilin, A., Bichoutskaia, E., Gimenez-Lopez, M. C., Chamberlain, T. W., Rance, G. A., Kuganathan, N., Biskupek, J., Kaiser, U., & Khlobystov, A. N. (2011). Self-assembly of a sulphur-terminated graphene nanoribbon within a single-walled carbon nanotube. Nature Materials, 10(9), 687-692.
Kuzmany, H., Shi, L., Pichler, T., Kürti, J., Koltai, J., Hof, F., & Saito, T. (2015). The origin of nondispersive Raman lines in the D‐band region for ferrocene@ HiPco SWCNTs transformed at high temperatures. Physica Status Solidi (B), 252(11), 2530-2535.
Kuzmany, H., Shi, L., Kürti, J., Koltai, J., Chuvilin, A., Saito, T., & Pichler, T. (2017). The growth of new extended carbon nanophases from ferrocene inside single‐walled carbon nanotubes. Physica Status Solidi (RRL)–Rapid Research Letters, 11(8), 1700158.
Linck, M., Hartel, P., Uhlemann, S., Kahl, F., Müller, H., Zach, J., Haider, M., Niestadt, M., Bischoff, M., Biskupek, J., Lee, Z., Lehnert, T., Börrnert, F., Rose, H., & Kaiser, U. (2016). Chromatic aberration correction for atomic resolution TEM imaging from 20 to 80 kV. Physical Review Letters, 117(7), 076101.
Börrnert, F., & Kaiser, U. (2018). Chromatic-and geometric-aberration-corrected TEM imaging at 80 kV and 20 kV. Physical Review A, 98(2), 023861
Borin Barin, G., Fairbrother, A., Rotach, L., Bayle, M., Paillet, M., Liang, L., Meunier, V., Hauert, R., Dumslaff, T., Narita, A., Müllen, K., Sahabudeen, H., Berger, R., Feng, X., Fasel, R., & Ruffieux, P. (2019). Surface-synthesized graphene nanoribbons for room temperature switching devices: substrate transfer and ex situ characterization. ACS Applied Nano Materials, 2(4), 2184-2192.
Kuzmany, H., Imhoff, E. A., Fitchen, D. B., & Sarhangi, A. (1982). Frank-Condon approach for optical absorption and resonance Raman scattering in trans-polyacetylene. Physical Review B, 26(12), 7109.
Vandescuren, M., Hermet, P., Meunier, V., Henrard, L., & Lambin, P. (2008). Theoretical study of the vibrational edge modes in graphene nanoribbons. Physical Review B, 78(19), 195401.
Talirz, L., Ruffieux, P., & Fasel, R. (2016). On‐surface synthesis of atomically precise graphene nanoribbons. Advanced Materials, 28(29), 6222-6231.
Skowron, S. T. et al. Chemical reactions of molecules promoted and simultaneously imaged by the electron beam in transmission electron microscopy. Acc. Chem. Res. 50, 1797–1807 (2017).